Wellbores may be drilled to locate and produce hydrocarbons. Typically, a wellbore is formed by advancing a downhole drilling tool having a drill bit at one end into the ground. As the drilling tool is advanced, drilling mud is pumped from a surface mud pit through a passage or passages in the drilling tool and out the drill bit. The mud exiting the drill bit flows back to the surface to be returned to the mud pit and may be re-circulated through the drilling tool. In this manner, the drilling mud cools the drilling tool, carries cuttings and other debris away from the drilling tool, and deposits the cuttings and other debris in the mud pit. As is known, in addition to the cooling and cleaning operations performed by the mud pumped into the wellbore, the mud forms a mudcake that lines the wellbore which, among other functions, reduces friction between the drill string and subterranean formations.
During drilling operations (i.e., advancement of the downhole drilling tool), communications between the downhole drilling tool and a surface-based processing unit and/or other surface devices may be performed using a telemetry system. In general, such telemetry systems enable the conveyance of power, data, commands, and/or any other signals or information between the downhole drilling tool and the surface devices. Thus, the telemetry systems enable, for example, data related to the conditions of the wellbore and/or the downhole drilling tool to be conveyed to the surface devices for further processing, display, etc. and also enable the operations of the downhole drilling tool to be controlled via commands and/or other information sent from the surface device(s) to the downhole drilling tool.
One known wellbore telemetry system 100 is depicted in FIG. 1. A more detailed description of such a known system is found in U.S. Pat. No. 5,517,464, which is incorporated by reference herein in its entirety. With reference to FIG. 1, a drilling rig 10 includes a drive mechanism 12 to provide a driving torque to a drill string 14. The lower end of the drill string 14 extends into a wellbore 30 and carries a drill bit 16 to drill an underground formation 18. During drilling operations, drilling mud 20 is drawn from a mud pit 22 on a surface 29 via one or more pumps 24 (e.g., reciprocating pumps). The drilling mud 20 is circulated through a mud line 26 down through the drill string 14, through the drill bit 16, and back to the surface 29 via an annulus 28 between the drill string 14 and the wall of the wellbore 30. Upon reaching the surface 29, the drilling mud 20 is discharged through a line 32 into the mud pit 22 so that rock and/or other well debris carried in the mud can settle to the bottom of the mud pit 22 before the drilling mud 20 is recirculated.
As shown in FIG. 1, a downhole measurement while drilling (MWD) tool 34 is incorporated in the drill string 14 near the drill bit 16 for the acquisition and transmission of downhole data or information. The MWD tool 34 includes an electronic sensor package 36 and a mud pulse or mudflow wellbore telemetry device 38. The mudflow telemetry device 38 can selectively block or partially block the passage of the mud 20 through the drill string 14 to cause pressure changes in the mud line 26. In other words, the wellbore telemetry device 38 can be used to modulate the pressure in the mud 20 to transmit data from the sensor package 36 to the surface 29. Modulated changes in pressure are detected by a pressure transducer 40 and a pump piston sensor 42, both of which are coupled to a processor (not shown). The processor interprets the modulated changes in pressure to reconstruct the data collected and sent by the sensor package 36. The modulation and demodulation of a pressure wave are described in detail in commonly assigned U.S. Pat. No. 5,375,098, which is incorporated by reference herein in its entirety.
In addition to the known mud pulse telemetry system 100 depicted in FIG. 1, other wellbore telemetry systems may be used to establish communication between a downhole toot and a surface unit. Examples of known telemetry systems include a wired drill pipe wellbore telemetry system as described in U.S. Pat. No. 6,641,434, an electromagnetic wellbore telemetry system as described in U.S. Pat. No. 5,624,051, an acoustic wellbore telemetry system as described in published PCT Patent Application No. WO2004085796, all of which are hereby incorporated by reference herein in their entireties. Further examples using data conveyance or communication devices (e.g., transceivers coupled to transducers or sensors) have also been used to convey power and/or data between a downhole tool and a surface unit.
Despite the development and advancement of wellbore telemetry devices in wellbore operations, there remains a need for additional reliability and wellbore telemetry capabilities for wellbore operations. As with other many other wellbore devices, wellbore telemetry devices sometimes fail. Additionally, the power provided by many known wellbore telemetry devices may be insufficient to power desired wellbore operations. Attempts have been made to use two different types of mud pulse telemetry devices in a downhole tool. In particular, each of the different mud pulse telemetry devices is typically positioned in the downhole tool and communicatively linked to a different, respective surface unit. Such wellbore telemetry tools have been run simultaneously and non-simultaneously and at different frequencies. Attempts have also been made to develop dual channel downhole wellbore telemetry for transmitting data streams via communication channels to be interpreted independently as described in U.S. Pat. No. 6,909,667.